1. Field of the Invention
The present invention is directed to a cooling device foran x-ray radiator that has an x-ray source arranged in a housing filled with coolant.
2. Description of the Prior Art
Approximately 99% of the electrical energy utilized in the generation of x-rays with an x-ray source is converted into thermal energy. The heat arising in the operation of the x-ray source usually must be eliminated from the x-ray source in some manner in order to be able to operate the x-ray source over a longer time span for radiological exposures of a subject. This is particularly required when high x-ray power is needed as, for example, in computed tomography or angiography.
The cooling of known x-ray radiators having a rotating anode x-ray tube ensues such that the heat arising during operation of the rotating anode x-ray tube is intermediately stored in the anode dish and is transferredxe2x80x94mainly by thermal radiationxe2x80x94to a coolant and insulating oil contained in the housing of the x-ray radiator. The coolant and insulating oil usually circulates in a closed circulation loop through the housing of the x-ray radiator and a heat exchanger that is thermally conductively connected to the x-ray radiator, that transfers the heat absorbed from the cooling and insulating oil to, for example, ambient air. When the maximally allowed temperature of the cooling and insulating oil is reached during operation of the x-ray radiator, this leads to relatively long, forced pauses wherein the x-ray radiator cannot be operated.
X-ray radiators of this type are disclosed, for example, in German OS 196 12 698 and German OS 197 41 750. Each of these x-radiators has a housing filled with a coolant wherein a rotating bulb tube is arranged and that is charged with the coolant for cooling. The coolant preferably flows through the housing and a heat exchanger connected to the x-ray radiator.
An object of the present invention is to provide a cooling device of the type initially described which can be operated over a longer time span.
This object is inventively achieved in a cooling device for an x-ray radiator that has an x-ray radiation source arranged in a housing filled with coolant, at least one phase change store, referred to below as a PCM store, provided with a phase change material (PCM) and firmly conductively connected to the x-ray radiator. A PCM store is characterized in that the phase change material, for example paraffin or salt, goes through a phase conversion at a specific limit temperature, this being at approximately 54xc2x0 C. for paraffin and approximately 72xc2x0 C. for salt. The temperature of the phase change material remains practically constant during the phase change, which ensues upon introduction of energy, since the supplied energy is required for the phase conversion. The energy supplied for the phase conversion is thereby intermediately stored in the PCM store and is turn released upon reversal of the phase conversion. An increase in the temperature of the phase change material ensues again upon further application of energy only after the phase conversion of the phase change material, for example from the solid into the fluid aggregate state. Inventively, such a PCM storexe2x80x94as a component of a cooling device for an x-ray radiatorxe2x80x94stores the thermal energy generated during operation of the x-ray radiator by means of its phase change material undergoing a phase conversion when its characteristic limit temperature is exceeded. Dependent on the volume and the nature of the phase change material of the PCM store, the temperature of the coolant of the x-radiator can be kept nearly constant over a certain time span despite the heat continuously arising during generation of x-rays. Compared to conventionally cooled x-ray radiators, the rise in the temperature of the coolant is retarded, so that a longer operating duration of the x-ray radiator is enabled.
U.S. Pat. No. 5, 507,337 discloses a heat pump and air-conditioning system that has a heat store provided with a phase change material.
In one version of the invention the PCM store is arranged in the coolant of the x-ray radiator. In this way, the PCM store can absorb thermal energy for the phase conversion of its phase change material directly from the coolant.
In other embodiments of the invention, the cooling device has a heat exchanger that is thermally conductively connected to the x-ray radiator and/or a storage vessel that is thermally conductively connected to the x-ray radiator that respectively contain at least one PCM store. The heat exchanger as well as the storage vessel increase the cooling capacity of the cooling device, whereby the heat of the x-ray radiator generated during operation thereof is transferred to and intermediately stored in the PCM store of the heat exchanger, preferably arranged at some distance from the x-ray radiator and/or of the storage vessel. In addition to the additional cooling of the x-ray radiator due to the output of heat of the heat exchanger and/or of the storage vessel, for example to the ambient air, this has the advantage that the energy being released in operating pauses of the x-ray radiator wherein the reverse phase conversion occurs, is not transferred only to the coolant of the x-radiatorxe2x80x94as in the case of the arrangement of the PCM store in the housing of the x-radiatorxe2x80x94but also is transferred to the air surrounding the heat exchanger and/or the storage vessel, resulting in the cooling and insulating oil of the x-radiator reaching normal temperature faster.
In versions of the invention the housing of the x-ray radiator and of the heat exchanger and/or the housing of the x-ray radiator and the storage vessel are connected to one another via lines such that the coolant can circulate through the housing of the x-ray radiator and the heat exchanger and/or through the housing of the x-ray radiator and through the storage vessel. Due to the circulation of the coolant, a more efficient heat transfer ensues from the x-ray radiator to each of the PCM stores arranged in the heat exchanger and/or in the storage vessel.
Since phase change materials exhibit only slight thermal conductivity, for which reason heat is absorbed and in turn emitted only slowly by phase change materials, in a preferred embodiment of the invention the PCM store is a matrix formed of a highly thermally conductive material into which the phase change materia is introduced. The limit temperature for the phase conversion of the thermally conductive material of the matrix is thereby so high that no phase conversion of the material ensues given utilization of the matrix in a cooling device for an x-ray radiator. As a result of introducing the phase change material into a matrix composed of a highly thermally conductive material, high charging and discharging currents can be advantageously achieved in the absorption or emission of heat by the PCM store. Moreover, no changes in volume of the PCM store occur upon temperature increases given an introduction of the phase change material into such a matrix, thereby simplifying the design of such a PCM store. Further, the introduction of the phase change material into a matrix causes the charging and the discharging temperature for the PCM store, i.e. the limit temperature at which the phase conversion occurs in the two directions, to be substantially the same.
It has also proven advantageous that the shape and size of a PCM store having a matrix can be very flexibly designed, so that an arrangement of the PCM store at arbitrary locations of the cooling device is possible with an appropriate design.
According to one version of the invention, the matrix is preferably formed of graphite.